The net result a simple example

How then do we assess the net GHG balance after a change in tillage intensity? We explore this question by presenting an example from semiarid agriculture like that in the northern Great Plains of North America. The estimates may not apply directly to cropping systems elsewhere, but the approach and the inherent insights derived from such an exercise may be more broadly applicable.

As shown in Table 5.2, the 30 years after adoption of NT are divided into three 10-year phases: an initial decade where soils are gaining carbon and N2O emissions are influenced by higher fertilizer N requirements; a second decade where carbon accumulation continues but at a lower rate, and N2O emissions subside as fertilizer N inputs are reduced; and a third decade where soil carbon is assumed to have reached a new steady state, and the NT system is mature.

Mean non-renewable energy inputs for a continuous spring wheat rotation under CT and NT management were calculated from values presented by Zentner et al. (1998). For this example, fertilizer N rates on NT were increased by 10% during the first decade to account for the additional nitrogen immobilized, and were equivalent to CT thereafter. Fertilizer-induced emissions (FIE) of N2O were calculated assuming that 1.25% of applied nitrogen is emitted as N2O (IPCC, 1997). Recent research suggests that N2O emissions tend to be lower from NT compared to tilled soils for the semiarid region of western Canada (Lemke et al., 1999; Helgason et al., 2005; Malhi et al., 2006); with these and other data (Lemke, 2006), we assumed that emissions decrease by 20%. Mean CH4 consumption rates were assumed to be 1.6 kg CH4/year for CT and 2.0 kg CH4/year for NT (Six et al., 2002). Because it takes time for rates to adjust, we assumed an intermediate value (1.8 kg CH4/ year) for NT during the first decade. Soil carbon was assumed to increase by an overall average of 320 kg/ha/year (VandenBygaart et al., 2003) for the first 20 years, with no further increases thereafter, but we assumed that rates were higher in the first decade (420 kg/ha/year) than in the second (220 kg/ha/year).

In our example (Table 5.2), emissions from fuel and machinery are slightly lower

Table 5.2. Sample budget of CO2 equivalents for a hypothetical site in the cool semiarid region of western Canada before and after adopting no-till (NT) farming practices.

Emissions (kg CO2e/ha/year)

Table 5.2. Sample budget of CO2 equivalents for a hypothetical site in the cool semiarid region of western Canada before and after adopting no-till (NT) farming practices.

from NT compared to CT systems, but this is more than offset by increased emissions associated with herbicides and fertilizers. Excluding changes in direct and indirect N2O emissions and soil carbon status, emissions actually increase slightly under NT during the first phase, but decrease slightly during the last two phases, resulting in a 30-year mean that is essentially equal to CT.

While the increased amount of fertilizer N used on NT during the first phase results in more FIE of N2O, we assumed for this example that the proportion of nitrogen lost as N2O from NT is less than from CT, resulting in a net reduction of emissions. In addition to changes to SOC, over a 30-year period NT would contribute ~1.0 t CO2e/ha less to the atmosphere than CT. Increases in SOC would be an additional net benefit. After 30 years, in this hypothetical illustration, NT would have removed more than 7 t CO2e/ha while its CT counterpart would have produced more than 16 t CO2e/ha.

The influence of NT adoption on overall CO2e emissions will be site-specific. Converting from CT to NT farming in the cool subhumid and semiarid regions of western Canada, particularly if pulse crops are included in rotation, will probably result in lower CO2e emissions from fossil fuel use, similar or lower N2O emissions, similar or higher CH4 consumption rates and modest gains in soil carbon status. These changes are 'synergistic', reducing overall CO2e emissions. In wetter regions, adopting NT may reduce fossil fuel use, but N2O emissions may be markedly increased. If an increase in soil carbon status occurs, overall CO2e emissions may still be lower on NT compared to CT, but this benefit would vanish over time as soils reach a new steady state. In some cool humid regions such as eastern Canada, where N2O emissions are likely to increase but soils do not appear to sequester carbon after adoption of NT (VandenBygaart et al., 2003), NT practices would likely increase overall CO2e emissions compared to CT. Clearly, the influence of NT on N2O emissions at a given site is a prominent factor in determining how this practice affects overall, long-term emissions of GHGs.